The assessment of acutely ill patients is a complex process involving evaluation of the patients numerous physiological systems, e.g. the pulmonary, metabolic, renal and circulatory systems. Much of the information necessary for this evaluation comes from analysis of the patients' blood. Blood samples can be taken from both arteries and veins. Arterial blood can be sampled either by placing an arterial catheter or cannula in the patient, or by performing an arterial puncture with a needle. Venous blood can be sampled from a cannula or a venous puncture at the periphery (peripheral venous blood); from a catheter placed in superior venal cava (central venous blood), or from a pulmonary arterial catheter placed in the pulmonary artery (mixed venous blood).
Placements of venous and arterial catheters are invasive procedures and generally restricted to high dependency departments. In addition catheterisation, cannulation or puncture of the arteries instead of the veins increases the risk of complications such as infection, hemorrhage, bleeding, thrombosis, emboli, neurological damage or pseudo-aneurysm formation. Sampling of arterial blood by arterial puncture is generally considered a more difficult procedure than sampling of venous blood through a venous puncture. Consequently, the routine sampling of arterial blood is generally restricted to high dependency environments. In other wards where patients are acutely ommited e.g. cardiology, abdominal surgery, thoracic surgery and medicine, routine sampling of peripheral venous blood is most common.
Many of the measurements taken from the blood, and used to assess the patient state, are similar in the venous and arterial blood samples. These included the electrolytes and such as sodium (Na), potassium (K), the haemoglobin concentration (Hb) and the concentration of abnormal forms of haemoglobin (e.g. carboxyhaemoglobin (COHb), methylhaemoglobin (MetHb)). However, the acid-base status of arterial and venous blood is not the same, regardless of the site of sampling. The acid-base status refers, in general, to the following measurements in blood: the pH, the pressure of oxygen (pO2), the pressure of carbon dioxide (pCO2), the bicarbonate concentration (HCO3), the concentration of base higher than a reference condition (base excess (BE)), the concentration of bicarbonate at a reference pCO2 (standard bicarbonate SBC), the oxygen pressure (pO2) and the saturation of haemoglobin with oxygen (SO2) with pO2 and SO2 often being referred to as the oxygenation status of blood. The variation in acid-base status between arterial and venous blood is due to oxygen removal from the blood and carbon dioxide addition due to metabolism at the tissues. In addition in patients with circulatory or metabolic abnormalities, the production of strong acid at the tissues due to anaerobic metabolism may also modify the acid-base status.
The acid-base status of arterial blood is used to assess the patients' respiratory and metabolic state. It has been argued (Adrogue et al., 1989a, 1989b; Brandi et al., 1995; Radiometer 1997) and to a large extent clinically accepted that venous blood samples are not adequate for assessing the acid/base and respiratory state of patients. This is thought to be particularly true for peripheral venous samples which “are not recommended for blood gas analysis as they provide little or no information on the general status of the patient” (Radiometer 1997).
In the intensive care unit placement of arterial catheters is routine practice and an assessment of the acid-base status can be obtained from the arterial blood. In some other hospital departments e.g. pulmonary medicine, or nephrology, arterial blood gases are also measured. However in other wards admitting acutely ill patients, e.g. cardiology, abdominal surgery, thoracic surgery and medicine, arterial samples are not usually taken. Usually a peripheral venous sample is taken and analysed in a central laboratory. The sample is usually taken aerobically, i.e. no attempt is made to ensure that pO2 and pCO2 remain constant during the sample procedure. Only a small amount of information concerning the acid-base status of the patient is measured in this sample i.e. the standard bicarbonate, SBCv, and haemoglobin Hbv. Other acid base parameters pHv, carbon dioxide pressure (pCO2v), base excess (BEv), oxygen saturation (SO2v) and oxygen pressure (PO2v) are not measured, and if measured would probably not reflect the true values of venous blood at this sample site given the aerobic nature of the sample.
U.S. Pat. No. 6,334,065 describes a pulse oximeter providing simultaneous and non-invasive oxygen status at multiple sites of a patient. The pulse oximeter described measures both arterial and venous oxygen saturation at any specific tissue site of the patient. It is mentioned that a corresponding computation of arterial minus venous oxygen saturation is advantageous for oxygen therapy patients. However, also as mentioned, the pulse oximeter is purely noninvasive in its way of functioning limiting the values capable of being derived.
U.S. Pat. No. 3,874,850 describes an apparatus being an automatic blood sample analyzer for automatically measuring one or more unknown data or parameters of the blood samples.
Based on the values measured, the apparatus comprises means for calculating a number of other parameters including acid-base status of the blood sample. The analyzer may also comprise means for photo-metrically measuring the hemoglobin contents of samples of blood. There is no computation of arterial blood values based on venous blood samples.